This invention relates to an oxygen sensor for detecting a concentration of oxygen contained in an exhaust gas of a motor vehicle, to control, for example, the air-fuel ratio of a mixture supplied to the vehicle engine.
Conventionally, oxygen sensors are used to control the air-fuel ratio for motor vehicles. Oxygen sensors of this type comprise an element which is made of a solid electrolyte capable of conducting oxygen ions and which has a first surface exposed to a reference gas having a constant concentration of oxygen, usually the atmosphere, and a second surface exposed to a gas of which the oxygen concentration is to be detected, i.e., the exhaust gas of a motor vehicle. The first and second surfaces are respectively provided with first and second porous electrodes both having gas permeability. Thus, the oxygen sensor forms a so-called concentration cell.
During operation of the oxygen sensor, when the concentration of oxygen contained in the exhaust gas, i.e., the partial pressure of oxygen, becomes substantially 0, that is, when the air-fuel ratio of a mixture supplied to the vehicle engine becomes smaller than the stoichiometric ratio and thus enriches the mixture, a large electromotive force is produced between the first and second electrodes of the oxygen sensor. Therefore, by detecting the voltage produced between the first and second electrodes, it is possible to determine whether or not the air-fuel ratio of the mixture is equal to the stoichiometric ratio, namely, whether the mixture is rich or lean. As a result, the air-fuel ratio of the mixture can be maintained at the stoichiometric ratio by controlling the quantity of fuel supplied to the engine from a fuel supply device in accordance with a voltage signal from the oxygen sensor.
When the air-fuel ratio of the mixture is equal to the stoichiometric ratio, the amount of carbon monoxide (CO) and hydrocarbon (HC) and the amount of nitrogen oxides (NOx) contained in the exhaust gases are small. This ratio is desirable for reducing of air pollution.
The above-mentioned conventional oxygen sensors are generally poor in responding to oxygen concentration changes, and are particularly disadvantageous, when the air-fuel ratio of the mixture changes to a lean (higher) side from a rich (lower) side after acceleration or high-load operation of a vehicle, because such a change to the lean side cannot be quickly detected. As the mixture becomes rich, the amount of carbon monoxide in the exhaust gas increases and the amount of carbon monoxide that is absorbed into the electrode exposed to the exhaust gas is increased, whereby the exhaust gas-side electrode is poisoned by carbon monoxide, If the electrode is poisoned, molecules of oxygen which are contained in the exhaust gas, cannot reach the electrode quickly enough. In such a case, when the air-fuel ratio of the mixture changes to a lean side, the oxygen sensor remains temporarily unable to detect such a change.
If a delay occurs in the detection of oxygen concentration in an exhaust gas by the oxygen sensor, as mentioned above, the fuel supply device erroneously adjusts an amount of the fuel supplied to the engine so that the air-fuel ratio of the mixture is further changed to a lean side. As a result, an excess of oxygen molecules exist in the exhaust gas and the temperature of the exhaust gas rises. Furthermore, a so-called lean spike occurs and the amount of nitrogen oxides in the exhaust gas increases.